Push-pull amplifier



m. 15, 1935. w. s. -BARDEN 2,017,199

PUSH- PULL AMPLIFIER Filed July 2'7, 1932 INVENTOR WILLIAM s. BARDEN ATTORNEY Patented Oct. 15, 1935 PATENT OFFICE PUSH-PULL AMPLIFIER William S. Barden, Stapleton, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application July 2'7, 1932, Serial No. 024,939

4 Claims.

My present invention relates to amplifier circuits, and more particularly to push-pull amplifiers of the class A and class B types.

One of the main objects of my present inven- 5 tion is to provide a push-pull power amplifier arrangement of the class A and class B type wherein not less than zero bias is utilized for the control electrodes of the push-pull tubes.

Another important object of the present inven tion is to provide a push-pull class A zero bias pentode tube arrangement which operates milirectly out of a power detector, the impedanchs between the output circuit of the detector and the common input circuit of the push-pull tubes 15. being designed in such a manner that maximum operating efiiciency is obtained.

Another important object of the present invention is to provide a push-pull class B amplifier arrangement wherein the control grids of the push-pull tubes are maintained at zero bias, and

the impedances between the power detector output and the input of the push-pull tubes are so matched that distortion due to grid current in the push-pull tubes is maintained at a minimum.

Another object of the present invention ,is to provide a power detector stage comprising a pair of tubes connected in parallel, the output of the power detector stage being arranged for connection to the input circuit of a push-pull class A zero bias pentode arrangement, or if desired, to the input circuit of a push-pull class B zero bias arrangement.

Still another object of the present invention is to provide a radio receiver embodying a power detector and a reproducer, and a single power amplifier stage between the output of the power detector and the reproducer, the power amplifier stage comprising a push-pull circuit including a pair of pentode tubes which operate as a class A zero bias push-pull circuit, the power output of the push-pull stage being twice the power output available with but a single pentode stage, third harmonic distortion in the output of said power amplifier stage being minimized, and low frequency degeneration being eliminated in addition to the aforementioned desirable results.

And still other objects of the present invention are to improve generally the simplicity and efiiciency of power amplifier circuits, and to particularly provide a push-pull power amplifier circuit which is not only durable, reliable and economical in operation, but economically manufactured and installed in a radio receiver.

The novel features which I believe to be char- 65, acteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have 5 indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing,

-Eig. 1 diagrammatically shows a conventional 10 radio receiver employing the present invention operati 1g as a class A push-pull amplifier,

Fig. graphically illustrates the operation of the invention shown in Fig. 1,

Fig. 3 shows a modified form of push-pull cirl5 cuit operating as a class B zero bias stage,

Fig. 4 graphically illustrates the operation of the modification shown in Fig. 3 and Fig. 5 shows a modified form of power detector circuit adapted for utilization with the present 20 invention.

Referring now to the accompanying drawing wherein like circuit elements in the different figures are designated by similar reference characters, there is shown in Fig. 1 in diagrammatic 25 manner a conventional type of radio receiver which comprises a grounded antenna circuit A, G. The circuit is coupled to any well known type of multi-stage radio frequency amplifier I, the output of the amplifier being coupled, as at M, to the 30 tunable input circuit of a detector tube 2. The said tunable input circuit includes a variable tuning condenser 3.

It is to be clearly understood that this variable tuning condenser may be mechanically uni-con- 5' trolled with one or more variable tuning condensers disposed in the usual and well known manner in the radio frequency amplifier stages. The detector stage, including the tube 2, operates on the plate rectification principle, negative bias 40 being provided for the grid of tube 2 by means of a bias resistor l, shunted by the usual radio frequency by-pass condenser 5, the resistor 4 being disposed in the negative leg of the indirectly heated cathode of the tube 2.

The current source B provides proper positive potential for the anode of tube 2, the positive terminal of source B being connected through the primary coil 6 to the anode of tube 2, while the negative terminal of source B is connected to the 50 low potential terminal of the resistor 4. A radio frequency by-pass condenser 1, having a magnitude of about 0.002 micro-farad is shunted across the coil 6, the source B and the resistor 4, and the resistor 4 is preferably given a magnitude of about 5,000 ohms. In order to operate the detector tube as a power detector, the source B is adjusted to apply a potential of about 250 volts to the anode of tube 2.

The tube 2 may be a 227 type tube and have a plate resistance r of about 10,000 ohms. The audio frequency coupling transformer 8, which includes the primary coil 6, has its secondary coil 9 connected in push-pull manner to the grids of pentode tubes Ill and I I, the midpoint of coil 9 being connected to the junction of the cathodes of tubes I0 and II in the usual push-pull manner. The current source B has its negative terminal connected to the common junction of the cathodes of tubes III and II, and has its positive terminal connected to the midpoint of primary coil I2 of the output transformer I3 in the usual manner to provide a push-pull output circuit.

The source B is adjusted to apply a potential of 250 volts to each of the anodes of tubes I0 and II, and the screen grids of each of tubes I0 and I I are connected to source B so that each screen has a potential of about 116 volts applied to it. The secondary coil I4 of the output transformer I3 is adapted to be connected to any desired type of reproducer, such as an electro-magnetic, or electrodynamic, reproducer.

The tubes I0 and II, as stated heretofore, are of the pentode type, and each includes an auxiliary grid disposed between the screen grid and anode, which auxiliary grid is connected within the tube to the cathode. Each of tubes I0 and I I may be of the 247 type. It will be observed, special reference being had to Fig. 2, that the tubes I0 and II do not have the control grids biased with respect to the cathodes. Each pentode tube is biased for operation at the point or shown in Fig. 2 in connection with the full line Ip v. Eg characteristic curve A. With this zero bias of the grids of the pentode tubes, the pushpull power amplifier stage including tubes I0 and II operates as a class A push-pull amplifier utilizing zero grid bias.

Fig. 2 shows, in dotted line, the grid current curve G when the signals impressed upon the grids of tubes I0 and II begin to draw current. The resultant, or dynamic, plate current-grid voltage characteristic curve is shown in Fig. 2. For optimum operation it has been found that the turns ratio between the primary coil of transformer 8 and each half of the secondary coil 9 should be three.

In other words, a stepdown transformer, designed according to the aforementioned turns ratio, should be employed for matching the impedance of the output circuit tube 2 to the common input circuit of pentode tubes I 0 and I I. It will be observed that in Fig. 1 there has been shown a pushpull class A zero bias pentode power amplifier arrangement which works directly out of a standard power detector circuit with step-down transformer coupling, and works directly into a reproducer.

The following advantages are secured as a result of the utilization of the push-pull amplifier in Fig. 4, when such a push-pull class A zero bias arrangement is utilized in place of a single pentode power amplifier stage:

(1) Twice the usual output is available.

(2) For a given energy level the tubes are operated with less grid swing with appreciably less harmonic distortion. 7

(3) The absence of any biasing resistor in the common input circuit of the push-pull stage eliminates low frequency degeneration which is often present when insuiiicient by-passing is placed across the biasing resistor of a pentode. (It is interesting tonote that most commercial receivers have insufiicient by-passing at this point in the circuit because of the large amount of capacity necessary to do a good job and the increase in cost resulting therefrom.)

(4) The arrangement of the power detector stage and the push-pull zero bias arrangement lends itself very readily to the compact and highly economical midget type of radio receiver, and avoids the necessity of using negative grid bias in the audio frequency grid bias stage or stages.

Of course it is to be clearly understood that the circuit preceding the input of the power detector stage need not be a tuned radio frequency amplifier, but may be the conventional frequency changer, first detector or intermediate frequency amplifier circuit for a super-heterodyne receiver. In this case the tube 2 would operate as the second detector of the super-heterodyne receiver. The secondary of the transformer 8 must have a low impedance in order to minimize grid distortion, and it has been found that with the turns ratio mentioned heretofore, for the transformer 8, distortion due to the drawing of grid current will not occur. It should also be understood that if desired the tubes I0 and II may be operated as class B push-pull amplifiers by biasing the grids of tubes I0 and II to the cut-off point of curve A in Fig. 2.

In Fig. 3 there is shown a modified form of push-pull amplifier power circuit comprising the tubes I0, II. The transformer 8 is again to be understood as having its primary coil 6 coupled across the output electrodes of the power detector output,'while the secondary coil 9 has its opposite terminals connected to each pair of twin grids of tubes I0 and II Here again, the ratio tubes, and each is a double-grid, dual amplifier. With the two grids 20, 2| of each tube, tied together, the amplification factor is so high that little plate current fiows at zero bias. Hence, it is unnecessary to supply a bias voltage, and the entire voltage output of the rectifier filter system is available.

The Ip V. Eg characteristic is shown in Fig. 4, and is designated by the reference letter A, the accompanying grid current curve being represented by the curve G, and the resultant dynamic characteristic curve is shown by the dotted line in Fig. 4. The source B should be adjusted so that a voltage of about 300 volts is applied to each of tubes I 0' and II, and it is to be noted that each of the grids I0 and II is conventionally represented. It is not believed necessary to show in detail the construction of tubes I0 and II, since the construction of these tubes is not a part of the present invention. It is merely necessary to point out that the plate current-grid voltage curve of a tube of this type is shown by the curve A of Fig. 4, and that the tube is constructed with two concentric grids, the inner one 2 I being coarse in comparison with the outer one 20.

7 It will, therefore, be noted that the push-pull power amplifier shown in Fig. 3 is basically similar to the arrangement shown in Fig. 1 in that zero grid bias is utilized in connection with the power amplifier tubes, it not being necessary to utilize negative grid bias in the audio amplifier stage. Furthermore, it should be observed that class B push-pull amplification is simultaneously secured in the modification shown in Fig. 3, with the desirable result that in battery operated receivers the plate current sources are conserved. Since the tubes l and II operate at zero bias, no resistors are necessary to supply this bias from plate or other currents, and degeneration effects due to such resistors are not present.

In Fig. 5 there is shown a modified form of driver stage which is adapted to precede either of the push-pull stages shown in Fig. l or 3. In this type of power detector stage a pair of tubes 30 and 3! are connected in parallel, a common grid biasing source 32 being utilized to bias the grids of the tubes 30 and 3|. A common anode potential source 33 is utilized to maintain the anodes of these two tubes at the proper high positive potential required for power detection purposes. The usual radio frequency by-pass condenser 34 is connected across the primary coil 6 and source 33, and it should be clearly understood that the secondary coil 9 of the transformer 8 is connected to the input electrodes of the pushpull stage shown in Fig. 1, or the input electrodes of the tubes shown in the power amplifier stage of Fig. 3. The advantage of using detectors in parallel in a power detector stage consists in the fact that the impedance of the detector output circuit is cut down, and therefore there is acquired less of a step-down ratio in connection with the transformer 8.

While in each of Figs. 1 and 3 it has been pointed out that the push-pull power amplifier stage is to be coupled to the output of a power detector stage, it is to be clearly understood that the present invention is not limited to the driving of the push-pull stage from a power detector stage. In fact, any driver stage may precede the push-pull stage, but in any case it is to be understood that the proper step-down ratio must be employed between the output of the driver stage and the input of the push-pull stage, since the grids of the push-pull stage are operated at zero bias, and distortion due to the drawing of grid current, in turn due to overloading of the push-pull input circuit, must beavoided for proper operation of the power amplifier stage.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made Without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a source of audio frequency energy at a predetermined potential level, a power amplifier stage including a pair of electron discharge tubes having their input electrodes connected in push-pull to the said source, and having their output electrodes connected in push-pull, the control electrodes of said pushpull tubes being connected to the cathodes thereof through a path of substantially low impedance to direct current whereby the control electrodes are at the same direct current potential as the cathodes throughout the operation of the stage, and means for coupling said source to the input electrodes of said push-pull tubes in such a manner that the potential level of the 5 audio frequency energy is decreased to a predetermined lower potential level, said push-pull tubes being of the pentode type and said pushpull stage operating as a class A amplifier.

2. In combination, a source of audio frequency 10 energy at a predetermined potential level, a power amplifier stage including a pair of electron discharge tubes having their input electrodes connected in push-pull to the said source, and having their output electrodes connected in push-pull, the control electrodes of said push-pull tubes being at the same direct current potential as the cathodes thereof throughout the operation of the stage, and means for coupling said source to the input electrodes of said push-pull tubes in such a manner that the potential level of the audio frequency energy is decreased to a predetermined lower potential level, each of said push-pull tubes having a plate current-grid voltage characteristic such that the push-pull stage operates as a class A amplifier and has a cut-off point at a point substantially negative with respect to zero grid bias.

3. In combination, a source of audio frequency energy at a predetermined potential level, a power amplifier stage including a pair of electron discharge tubes having their input electrodes connected in push-pull to the said source, and having their output electrodes connected in push-pull, the control electrodes of said push-pull tubes be- 3.5 ing at the same direct current potential as the cathodes thereof throughout operation of the stage, and means for coupling said source to the input electrodes of said push-pull tubes in such a manner that the potential level of the audio 4 frequency energy is decreased to a predetermined lower potential level, each of said push-pull tubes having a plate current-grid voltage characteristic such that the push-pull stage has a cut-off point at a point substantially negative with respect to zero grid bias, said coupling means including a step-down transformer which has a ratio of three between its primary turns and each half of its secondary turns.

4. In combination, a source of audio frequency energy at a predetermined potential level, a power amplifier stage including a pair of electron discharge tubes having their input electrodes connected in push-pull to the said source, and having their output electrodes connected in pushpull, the control electrodes of said push-pull tubes being at the same direct current potential as the cathodes thereof throughout the operation of the stage, and means for coupling said source to the input electrodes of said push-pull tubes in such a manner that the potential level of the audio frequency energy is decreased to a predetermined lower potential level, each of said push-pull tubes having a plate current-grid voltage characteristic such that the push-pull stage has a cut-ofi point 5 at a point substantially negative with respect to zero grid bias, said coupling means comprising a step-down transformer.

WILLIAM S. BARDEN. 

